The present invention relates to a head positioning control system for use in a hard disk drive for example, in which the positioning control operation of a magnetic head unit separated into a read head and a write head is performed on the basis of the servo data recorded in advance on a disk making up a storage medium.
In conventional hard disk drives (HDD), data are written into and read from a head (magnetic head) on a disk providing a storage medium.
Recently, a head unit separated into a read head and a write head has come to be used in which a MR (magnetoresistive) head is used as the read head in order to realize a high-density data recording. This type of head unit uses an inductive head as the write head, and the write head and the read head are separately mounted on a slider constituting a head unit body. The MR head has a high reproduction output characteristic, and therefore is most suitable as a read head for reading the data recorded in high density on the disk.
A multiplicity of concentric tracks are arranged on each data surface of the disk. Each track is divided into a plurality of data sectors. Further, servo areas having servo data recorded therein are arranged at predetermined intervals on each track. The servo data are roughly divided into track codes (cylinder codes) indicating track addresses for identifying each track and servo burst data (burst patterns A to D). The servo burst data include two-phase burst patterns A, B and C, D for detecting the head position on the track.
Once a target track to be accessed is determined, the head positioning control system (comprising a CPU as a main element) of the HDD first executes the seek control (or speed control) operation for moving the head to the target track using the track code read by the read head. With the approach of the head to the neighborhood of the target track, the head positioning control system executes the track following control operation in which the head (read head or write head) is set in position within the range of the target track (normally, by causing the track center to coincide with the head center) using the track code read by the read head and the servo burst data.
In the track following control operation, the CPU executes the positioning error calculation (a-b)/(a+b) using the amplitude values (assumed to be digital values a, b) of the burst patterns A, B read by the read head and thus calculates the positional information (the positional error or the following error) of the head with respect to the track center. Also, the CPU executes the positional error calculation (c-d)/(c+d) using the amplitude values c, d of the burst patterns C, D thereby to complement the discontinuity of the positional information based on the burst patterns A, B. In this way, the CPU uses the amplitude values c, d of the burst patterns C, D for determining the position of the head not existing on the target track and for detecting the direction in which the head is moved toward the track.
The positional information based on the positional error calculation (a-b)/(a+b), as shown in FIG. 9, undergoes a change along a solid line 80 in accordance with the head position along the track width within the range of a track N, for example. The CPU controls the head position so that the positional error value (a-b)/(a+b) finally becomes zero.
In the case where a read/write inductive head is used as a magnetic head, the CPU executes the position control operation based on the same positional information for both the read and write operations. In the head unit using a MR head as a read head and having separate read and write heads, on the other hand, the CPU is required to execute the positioning control of the read head at the time of the read operation and to execute the positioning control of the write head at the time of the write operation.
With the HDD, as shown in FIG. 8, a mechanism for driving the disk 1 and the head 3 is arranged in a housing 70. A rotary-type actuator (carriage) 6 makes up a drive mechanism for the head 3. The actuator 6 is a mechanism for moving the head 3 in radial direction of the disk 1 by the driving force of a voice coil motor 7. A single or a plurality of the disks 1 are fixed on a spindle motor 2 and rotated at high speed.
The use of this rotary-type actuator generates an angle (skew angle SA) between the normal to a write gap 5a and the tangential direction of disk rotation (corresponding to the track center line TC in the present case) when the write head (write gap 5a), for example, is set in position within the range of a track, as shown in FIG. 10. The skew angle SA changes with the position of the track in which the head exists. Further, since the read gap 4a of the read head is separated from the write gap 5a of the write head, the respective center positions are displaced along the track width and therefore an offset PE is caused. The offset PE changes with the track position of each head at the time of the read or write operation. Also, in this head unit, the manufacturing tolerance is another cause of a displacement of the read gap 4a and the write gap 5a along the track width for lack of a predetermined positional relation between them.
In the HDD, the positioning control is executed using the servo burst data (i.e., the burst patterns A, B) for finally setting the head at a designated position (within the tolerable range of the target track) on the disk. In the head positioning control operation for a head unit having separate read and write heads described above, it is necessary to absorb the displacement (offset PE) along the track width between the read gap 4a and the write gap 5a attributable to the variation in skew angle A and the manufacturing tolerance.
In other words, as shown in FIG. 7A, the read head is required to be positioned in such a manner that the read gap 4a may be included in the range of the data track TD (width of the write gap 5a) providing the data recording area at the time of data read operation. Specifically, the width of the write gap 5a is set wider than the width of the read gap 4a. Also, the center axis of the read gap 4a and the center axis of the write gap 5a are displaced from each other along the track width in the manufacturing process by predicting the change in the skew angle SA. Further, when positioning each head at the time of data read operation and data write operation, the corrective operation is performed to allow an offset along the track width. Specifically, in this offset-allowing corrective operation, as shown in FIG. 7B, the set point RP of the read head at the time of data read operation is located at a different position from the set point WP of the write head at the time of data write operation for the same target track.
For a high recording density of the HDD to be realized, it is also important to increase the track density (tracks per inch (TPI)) on the disk as well as to increase the track recording density (bits per inch (BPI)). In a method of increasing the track density with the write gap 5a set wider than the read gap 4a as described above, the decrease in the track pitch (the track width TW in FIG. 7A) is limited in order to suppress the crosstalks from adjacent tracks. In the case where the width of the write gap 5a (the data track width TD) is decreased in order to relax the limitation of the track pitch for suppressing the crosstalks, on the other hand, an increased offset is required at the time of head positioning control. This is liable to cause the burst patterns A, B to exceed the controllable range and forms a stumbling block to an increased track density.